Brake pressure estimation is a fundamental basis for modern brake control systems, especially roll stability control and conventional electronic stability control. Originally, antilock brake systems did not typically rely on brake pressure estimation. However, modern algorithms are increasingly relying on it. This is in part due to the fact that the algorithms are available with stability control implementations. Further, using brake pressure estimation to support antilock control can simplify arbitration between requests from competing controllers for antilock, traction and stability control. Some methods of estimating brake pressure use valve times and models of hydraulic components. However, these methods tend to suffer inaccuracies due to variation in components and operating conditions.
A fundamental part of antilock control is tire-road μ estimation. Tire-road μ, a coefficient of friction, estimation is commonly used to adjust thresholds for control logic (i.e., slip, acceleration), estimate vehicle deceleration for stabilizing reference speed calculations, change modes of control such as for split-μ control and μ-transition control. Current methods estimate tire-road μ for a vehicle by observing vehicle deceleration during antilock control. However, these methods require multiple control cycles and are subject to errors when road grades are involved. Further, there is a reduction in accuracy when less than all of the wheels are in antilock control. Additionally, these methods are tuned empirically and lead to distinct state changes in control.
There is a need for a more accurate, more robust brake pressure estimation that is continuous and seamlessly integrated into general vehicle braking control.